Easy questions - fun ICE motorcycle starter battery DIY

First project to learn make battery packs.

I'm want to tinker together a simple replacement for 12v 5Ah SLA motorcycle starter battery using 18650, bms and vruzend becuse i dont have spot welder. Can solder but I suck big time. Starter motor is ~400w - calculating 600w for margin/light bulbs before engine start etc, ie 50A max draw. Generator power is ~250w (?) regulated/rectified to stable voltage somewhere between 13,5v-14,5v.

I'm thinking 2,5Ah+ is enough because of peukert effect. I'm thinking get 3s 18650 and maybe 2p - because not sure 1p can deliver 50A burst 3-10 sec (when 30-35A cont). I'm thinking a BMS board with same charger/discharge leads (there are bms with separate charge and discharge), for simple plug and play - SLA compatibility is crucial requirement.

So:
3 or 6 (2p) of these https://eu.nkon.nl/rechargeable/18650-size/sony-konion-us18650vtc6-3120mah-30a-met-button-top.html
1 pack vruzend (better alternatives? dont have spot welder)
1 http://www.ebay.com/itm/12V-3S-100A-PCM-BMS-same-port-of-charge-discharge-balance-for-Li-ion-battery-/282449011803?hash=item41c345e45b:g5AAAOSwVm5ZAS3Y
1 large shrinkwrap havent found yet, or I just wrap in duct tape because yolo

Is this good - any recommendations or did I miss something?

Question 1: BMS - compatible with the cells?
Question 2: BMS - it wants charging voltage 12,6v - would it handle 13,5-14,5, or do I need to change to another BMS - which one? I can add an additional regulator between generator and harness but really would prefer just swap battery pack and not make incisions in cable harness. More capable bms much preferred even if cost is higher.
Question 3: Will I be able to charge this with regular SLA charger, and BMS cut off when full?
Question 4: Will this be a foolproof pack - ie. will BMS cut off and save cells when too low voltage, when fully charged, and balance and all that?
Question 5: Would 1p be enough? How many seconds on/off starter motor would be doable, for example 3 sec on, 10 sec off? If need multiple tries a cold morning.

Thanks! =)

1: yes
2: 12.6v is an absolute maximum for 3s liion. If you feed it more you are at real risk of a fire at some stage. Relying on a bms to cut off the charge when full is never a good idea with a constant over voltage supply. If it doesn't then you have a big problem. A 12.6v regulated supply plus bms would be much better.
3: no-as above plus liion hates trickle charging.
4: no definitely not.
5: I suspect but don't know for sure that you will draw more than 50a momentarily. I'd go 2p min.

Have you considered a 4S lifepo pack in a high discharge cell. At 14.4v it's almost a direct replacement for 12v sla. A big format cell like headway or 26650 could get you what you want. The charging issue is mostly solved ( still want a bms). It's probably a better option.

kdog said:

1: yes
2: 12.6v is an absolute maximum for 3s liion. If you feed it more you are at real risk of a fire at some stage. Relying on a bms to cut off the charge when full is never a good idea with a constant over voltage supply. If it doesn't then you have a big problem. A 12.6v regulated supply plus bms would be much better.
3: no-as above plus liion hates trickle charging.
4: no definitely not.
5: I suspect but don't know for sure that you will draw more than 50a momentarily. I'd go 2p min.

Have you considered a 4S lifepo pack in a high discharge cell. At 14.4v it's almost a direct replacement for 12v sla. A big format cell like headway or 26650 could get you what you want. The charging issue is mostly solved ( still want a bms). It's probably a better option.

Click to expand...

Thanks kdog! Thanks for suggesting another method - so I could get these:
https://bmsbattery.com/ebike-battery/405-4s-lifepo4-bms-for-4-cells-in-series-battery.html
https://hobbyking.com/en_us/zippy-flightmax-4200mah-4s1p-30c-lifepo4-pack.html alternatively 4x these: https://eu.nkon.nl/rechargeable/26650/a123-systems-anr26650m1b-a-grade-3-3v-a-grade.html

Like this - and i'm done?

If not, what would you recommend?

I'd go the A123 and bms option, better than Rc quality stuff. Do you know what V your bike puts out normally and at max output? I mean have you actually measured it? 14.5v would be ok but I wouldn't want it any higher.
Might pay to get a few other opinions more knowledgeable than mine bc I haven't done an auto battery before. I suspect with the 4S Lifepo option you'll just have a reduced cycle life from the constant charge but the rest will be fine.
One other thing. If the BMS turns off whilst you are riding, this may affect the V stability of the regulator, or could it even turn your bike off- dunno, just wondering!?